Heretofore, a silica-based coating material has been used in various fields. For example, in a semiconductor device, it has been used as an insulation film between a semiconductor substrate and a metal wiring layer such as an aluminum wiring layer, or between metal wirings. Further, it has been used as a PN junction semiconductor formed on a semiconductor substrate, or as a protective film for various elements such as a capacitor element, a resistor element, etc.
Further, when a metal wiring layer or the like is formed on a semiconductor substrate, irregularities are formed by the metal wiring layer or the like on the semiconductor substrate. If it is attempted to form a metal wiring layer or the like on such an irregular surface, breaking of wire is likely to result due to the difference in level of the irregularities. By forming a silica-based insulation film as mentioned above, on such an irregular surface formed by a metal wiring layer and various elements, a role of planarization is also performed.
With respect to the silica-based coating film used in the above-mentioned fields, it is common to form the silica-based coating film by a vapor phase growing method such as a CVD method or a sputtering method, or a coating method for forming the silica-based coating film by means of a coating composition for forming a silica-based coating film.
As a coating fluid to form such a silica-based coating film, a siloxane polymer has been used. A polyalkylsilsesquioxane precursor as one of precursors of such a siloxane polymer, is disclosed, for example, in JP-A-63-241076 or JP-A-3-126612. However, the conventional method is a method wherein an alkylalkoxysilane is hydrolyzed to form an alkylsiloxane polymer. In such a method, it is not easy to control the hydrolysis or the reaction for polycondensation. Further, if such a solution is highly concentrated to such an extent that a thick film can be formed by a single coating operation, the storage stability tends to be poor. Accordingly, in order to form a thick film, it has been common to carry out coating a few times for overcoating.
Further, a method wherein a polysilazane is thermally treated and used as a silicon oxynitride film (JP-A-62-88327), or a method wherein a polysilazane is thermally treated in a steam atmosphere and used as converted to a silicon oxide film. In such methods wherein a polysilazane is employed, it is possible to form a thick film by a single coating operation, but there is a problem such that ammonia or an amine will be generated during the thermal treatment and will contaminate the wiring material, etc.